1. Field of the Invention
The present invention relates generally to blast nozzles for removing adherent material such as paint, scale, dirt, grease and the like from solid surfaces by directing a stream of abrasive particles propelled by a combination of liquid and air against the substrate surface. In particular, the present invention is directed to a novel and improved blast nozzle in which high pressure water is injected coaxially into a stream of abrasive particles entrained in air internally in the blast nozzle to improve dust control without reducing blast cleaning productivity.
2. Description of the Prior Art
In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, or simply to degrease a solid surface such as surfaces contacting food or building structures which contain food serving or food processing operations, it has become common practice to use an abrasive blasting technique. In such process, abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Various abrasive blasting techniques have been utilized to remove the coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 1,000 psi and above, and a process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technique, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation.
A typical dry blasting apparatus as well as a wet blasting apparatus which utilizes highly pressurized air to entrain, carry and direct the abrasive blast media to the solid surface to be treated and high or low pressure water for dust control comprises a dispensing portion in which the blast media typically contained in a storage tank is entrained in highly pressurized air, a flexible hose which carries the air/blast media mixture to the blast nozzle and which allows the operator to move the blast nozzle relative to the surface to be cleaned and the blast nozzle which accelerates the abrasive blast media and directs same into contact with the surface to be treated. Water is added either internally in the blast nozzle and mixed with the air stream passing therethrough or a low pressure stream of water is provided externally of the blast nozzle and directed at the abrasive stream or surface to be treated so as to control dust. The blast nozzle is typically hand-held by the operator and moved relative to the targeted surface so as to direct the abrasive blast media across the entire surface to be treated.
The blast media or abrasive particles most widely used for blasting surfaces to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e., silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to non-soluble blast media such as sand, in particular, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, composite surfaces, plastics, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subsequent to blasting can be simply washed away without yielding environmental harm. Sodium bicarbonate, typically used as particles having average diameters of from about 50 to 1,000 microns, is more friable than sand. As the sodium bicarbonate media contacts the surface to be treated, small particles of the media are formed yielding a substantial amount of dust which invades the targeted area and closely surrounding environment, hindering the operator's vision of the targeted surface. Accordingly, it has become necessary to control the dust which is formed upon blasting with the very friable sodium bicarbonate blast media.
As expressed above, it is possible to control dust by mixing a stream of water with the air stream which propels the blast media. This has been accomplished by two distinct methods. In one method, the blast nozzle is provided with a water port in which water is injected into the blast nozzle to mix with the air stream and entrained blast media particles. This method has been very effective in controlling the dust of the particles subsequent to contacting the targeted surface. Unfortunately, the velocity of the media particles is reduced by the water and consequently, the productivity with respect to cleaning the targeted surface is substantially decreased by this method. Thus, defining performance of a blast nozzle as the rate in which a volume of coating is removed per time, injecting the water with the air stream which propels the blast media has greatly reduced the production rate relative to dry blasting.
Similarly, in high pressure water blasting, abrasives are added when necessary to increase the aggressiveness of the water stream. The effectiveness of the combined water and abrasive media stream is often wanting as it is difficult to add the abrasive particles moving at relatively low velocity with the high velocity water into a uniform mixture moving at the higher velocity.
A blast nozzle apparatus used to propel abrasive particles entrained in a mixed air and water stream against a target surface for cleaning is disclosed in afore-mentioned U.S. Pat. No. 4,817,342. In this patent, the abrasive which is entrained in a compressed air stream is mixed with high pressure, e.g. 1500 to 4000 psi, water prior to or at the inlet of a venturi-type blast nozzle. The venturi-type blast nozzle is one which contains a converging portion, a venturi orifice and a downstream diverging portion in which the velocity of the abrasive particles is imparted by the thermodynamic expansion of the gas (air). In the diverging section of such blast nozzles, gas velocities in excess of sonic velocity are attained. Unfortunately, improvements in the productivity of the wet blasting nozzle are not substantially improved by the addition of abrasive inasmuch as the high pressure water has a substantially higher velocity than the abrasive at the inlet of the blast nozzle and thus, uniform mixing of the abrasive with the water stream is not readily achieved.
An alternative method to control dust has been to direct a low pressure water stream externally from the blast nozzle at the targeted surface to control the dust which forms at the contact point. While this process has yielded improved productivity relative to the internally directed water stream, dust control is only slightly improved relative to dry blasting and substantially inferior to the process in which the water stream is directed at the abrasive stream internally in the blast nozzle.
An object of the present invention is to provide a blast nozzle useful in wet blasting wherein a particulate abrasive entrained in compressed air is mixed with a high pressure liquid stream in a manner which achieves increased cleaning or performance rates above comparable prior art devices.
Another object is to improve a process for wet blast cleaning by which a particulate abrasive is mixed with a high pressure water stream so as to achieve increased cleaning or performance rates.
In view of the advantages of utilizing sodium bicarbonate as a blast media as enumerated above, including water solubility to improve clean up, less harmful to the environment and useful to clean a wide variety of different surface types, still another object of the invention is to improve the processes and apparatus for using water soluble abrasive media such as sodium bicarbonate blast media in a manner to reduce the dust and, at the same time, maintain the productivity found in dry blasting.